Changes in the pH, EC, available P, SOC and TN stocks in a single rice paddy after long-term application of inorganic fertilizers and organic matters in a cold temperate region of Japan

Purpose The objective of this study was to determine the changes in the main soil chemical properties including pH, electrical conductivity (EC), available phosphorus (P), soil organic carbon (SOC) and total nitrogen (TN) stocks after long-term (31 years) additions of two types of organic matters—rice straw and rice straw compost, combined with NPK fertilizers in single rice paddy in a cold temperate region of Japan. Materials and methods A long-term experiment on combined inorganic fertilizers and organic matters in paddy rice cultivation began in May 1982 in Yamagata, northeastern Japan. After the 31st harvest, soil samples were collected from five treatments [(1) PK, (2) NPK, (3) NPK + 6 Mg ha −1 rice straw (RS), (4) NPK + 10 Mg ha −1 rice straw compost (CM1), and (5) NPK + 30 Mg ha −1 rice straw compost (CM3)] at five soil depths (0–5, 5–10, 10–15, 15–20, and 20–25 cm). Soil chemical properties of pH, EC, available P, SOC, and TN were analyzed. Results and discussion The pH decreased significantly only at the higher compost rate of 30 Mg ha −1 , while EC increased in all the organic matter treatments. Available P significantly increased in the CM1 and CM3 treatments by 55.1 and 86.4 %. The amounts of SOC stock increased by 67.2, 21.4, and 8.6 %, and soil TN stock by 64.1, 20.2, and 8.5 % in CM3, RS, and CM1, respectively, compared to NPK treatment. Conclusions Significant changes in soil properties were observed after 31 years of organic matter applications with reference to PK- and NPK-fertilized rice paddy soils. A significant decrease in pH was observed with the application of a high rate (30 Mg ha −1 ) of rice straw compost but not with the conventional rate of 10 Mg ha −1 . However, EC increased significantly relative to that of the PK- and NPK-fertilized plots in all the organic matter treatments. Available P significantly increased in the CM1 and CM3 treatments by 55.1 and 86.4 %. The amounts of SOC stock expressed as a percentage of total C applied to the soil were higher from 10 Mg ha −1 compost (28.7 %) than that from 6 Mg ha −1 rice straw (17.4 %), indicating a more effective soil organic C accumulation from rice straw compost than that from original rice straw.